Does any trainers in Pokemon Platinum own a Gardevoir and if so tell me?

No, there are no trainers in Pokemon Platinum who own a Gardevoir.

Gardevoir is a Pokemon that evolves from Kirlia, which in turn evolves from Ralts. You can find Ralts in the Grand Underground, but you'll have to find one yourself and train it up to evolve it into a Gardevoir.